I don't watch much TV generally, because I find, somewhat scarily, that my concentration span isn't up to it. My boyfriend, who before he met me was a regular square-eyes, has more or less given it up as a bad job these days, as apparently it is impossible to watch television properly when someone is ceaselessly inquiring, in an extremely irritating and indignant high-pitched piping voice, "Who's that, then? Why isn't he talking to that woman when he was snogging her two minutes ago? Where's that policeman going? Whose dog is that?" And that's just the Ten O'Clock News.

But all this has changed since the advent of the new six-part ITV series Bob And Rose, when for an hour each week I become a wide-eyed, mute zombie who could not be distracted from the screen even if the Appleton sisters climbed in naked through the window, covered in Nutella and carrying a game of Twister, and proceeded to play with it behind the sofa. My boyfriend was cynical at first, and explained away my new-found serenity by pointing out that I had a really soppy, girlie crush on Alan Davies, and that I even go "Ahhhh!" in the manner of a moron seeing a photo of a cuddly kitten in a Wellington boot when Davies appears in that building society ad.

It's true, I do and this has been chronically exacerbated by the recent confession of a friend that she slept with him a few years ago and that he was, not to put too fine a point on it, bloody brilliant. But I'm fascinated by Bob And Rose because it parallels the experience of three gay men of my acquaintance who have, almost overnight it seems, fallen for women.

The most notable of these is my friend, the brilliant young playwright Tim Fountain, who found himself directing the notorious Puppetry Of The Penis show on tour. After a few weeks of watching them being turned into the Taj Mahal and Mickey Mouse, Tim started to find penises curiously unappetising; meanwhile, the brilliant young actor-comedian Jackie Clune, a lesbian, who was also on the bill, began to find them mysteriously beguiling. The pair toppled into a rampant affair, and Tim then had the rather surreal experience of having to "come in" to his parents. His father, with supreme Yorkshire bluffness, retorted, "I told you this would happen when you hit your 30s!"

But Tim, Jackie, Bob and the others still describe themselves as gay - though Tim thinks he may have been "a lazy heterosexual". Not one of them, interestingly, will go near the B-word. And typically, even that old fraud David Bowie, who had a whole generation parroting "Everyone's bisexual!", has recently been banging on about what an effort it was to fancy men for all those years.

When did bisexuality stop being the belle of the ball and become the love that dare not speak its name? Since we realised that, with a few honourable exceptions, those who rally under its banner tend to be more than usually dim, dull and desperate. There were always clues, of course. Those friends of mine unfortunate enough to have been to university tell me that in the first week, when the various clubs and societies set out their stalls, there would always be a bisexual society stall with a banner bearing the words "TWICE THE FUN!" and the most miserable and nondescript creature manning it. Similarly, Hanif Kureishi, who famously advocated bisexuality because "you know when you go to a party you've got twice the chance of getting off with somebody", probably reckoned he sounded like Mr Sex, when in fact he came across as Dr Desperate. You wouldn't have twice the chance at all, in my opinion, because once word got around that you were an undiscriminating little slag who'd do it with literally anyone, most right-minded people would give you a wide berth.

No one thinks bisexuality is cool these days, with the possible exception of insecure young girls who snog other girls when drunk at parties, so their bored boyfriends will be titillated and won't dump them just yet. It really is something, like ambition or acne, that you should grow out of. And, paradoxically, it indicates a lack of sexual experience rather than an excess. I spent a good deal of my teenage years as a provincial virgin trying to convince any poor sucker who'd listen that I was bisexual, when in fact I didn't know which end was which. As Alan Bennett said, when asked if he was gay or straight, "That's like asking a parched man who's been crawling across the desert for three days whether he'd like Evian or Perrier."

Sexual sophistication is signalled by narrowing, not widening, our options; only the terminally naff believe otherwise. Like orgies and S&M before it, bisexuality has become a feast for the sex-starved suburban swinger. For instance, one Blackpool hotel offers Bisexual Weekends and even, with chronic joylessness, a Bisexual Xmas and New Year Special - every totem of concupiscent corn is present and correct, from Bucks Fizz Breakfast to parties on a theme of maids and butlers, teachers and schoolgirls, and sexy underwear. For some reason, this knowledge makes me want to weep.

In the end, it is the idiotic fact of wanting to identify as "bisexual" - a state that by its very nature should be about flexibility and fluidity - that makes smart people shy away from it. As Gore Vidal said in a rare moment of insight, people commit homosexual or heterosexuals acts - they are not homosexuals or heterosexuals. How much sillier, then, to call oneself a "bisexual" - even if the word didn't sound so much like bicycle. There's always been a perfectly serviceable word for someone who'd have sex with anything, even a Venetian blind, if it kept still long enough, and that word is "slut". Deal with it.